The present invention relates to a micro-Pirani thermal conductive vacuum sensing device with a new structure and to a circuitry design for temperature compensations, and more particularly, to a method for manufacturing such a vacuum meter.
The vacuum industry has been developed for a very long time. In scientific studies and industrial utilizations, the vacuum equipments are used widely. One essential device of the vacuum equipments is a meter for sensing the vacuum degrees. There are many vacuum meters in the market. The following table represents various pressure regime (in torr) of the well-known sensing meters. The pressure region most commonly used at the present is from 10.sup.-6 -10.sup.-5 torr to an atmospheric pressure. In general, the metering region of a single vacuum meter cannot cover the above pressure region. To obtain such a pressure region, it should use alternatively two vacuum meters.
TABLE 1 __________________________________________________________________________ Pressure Regions of Various Vaccum Meters __________________________________________________________________________ ##STR1## ##STR2## ##STR3## ##STR4## ##STR5## ##STR6## ##STR7## __________________________________________________________________________
At present, the commonly-used vacuum meters are vacuum meters of thermal conductivity type. The well-known thermal-conductive vacuum meters mainly comprise two kinds: Pirani vacuum meters and thermocouple vacuum meters. The conventional Pirani vacuum meters are designed according to the relationship of heat loss of a filament heated in vacuum and the vacuum pressure within a suitable-used region of the meter, i.e., the relationship when the mean-free path of molecules in a certain vacuum pressure is corresponding to the inner size of a cavity in which the heated article is installed. FIG. 16a and 16b show the heat-dissipation diagram and curves of a heated article (i.e. a heated filament 19 in this drawing) in vacuum, respectively. As shown in FIG. 16a, the heat loss of the filament 19 can be dissipated from (1) the solid leads of the filament (solid conduction, as indicated in the arrow 1); (2) the surface of the filament (radiation, as indicated in the arrow 2) and (3) in the form of vapor molecules (gase conduction, as indicated in the arrow 3). If the heat loss of the heated article dissipated in the form of vapor molecules is irrelevant to the vacuum pressure or is very small in comparison with the heat loss dissipated in the other two manners, the vacuum pressure can not be measured. In the heat-transfer of the vapor molecules, when the molecules strike the filament 19, a part of thermal energies of the filament 19 will be transferred to kinetic energies of the molecules and be taken away by the molecules. When the molecules with kinetic energies strike the wall of the cavity with a lower temperature, a partial kinetic energy of the molecules will be transferred outwardly via the wall of the cavity. More the number of the molecules striking the wall are, more the transferred heat is. Further, in a constant bias condition, the temperature changed accordingly of the filament is inversely proportional to the number of striking molecules. The change of the temperature results in the change of the resistance of the filament. Therefore, it can use a bridge circuit to measure the variation of the resistance of the filament and to calculate the density of molecules and then the vacuum pressure is measured therefrom.
The above thermal conductive vacuum meter, however, is effective only within certain pressure regions, which is relative with the construction of the filament and the cavity and the leads of the filament. FIG. 16b shows respective heat-transferring curves of the solid leads, the surface of the filament and the vapor molecules (A), (B) and (C). As shown in Curve (C), in the case of a high vacuum pressure, though the number of molecules striking the filament increases proportionally to the increase of the pressure, the mean-free path of molecules will be decrease inversely so that the heat loss can not be effectively transmitted outside of the wall of the cavity. It means that when the vacuum pressure exceeds a limited value, the variation of pressure will no longer influence the rate of gas transmission and at this time, the vacuum meter reaches its maximum limitation of measurement, which is determined by the ratio of the mean-free path of molecules and the distance between the meter and the wall of cavity. Further, in the case of an extremely low pressure (i.e., high vacuum degrees), the number of molecules will be largely decreased to result that the gas heat-transfer is smaller than the heat transfer from the leads and the surface of the filament. Therefore, the variation of temperature of the gas will be too small to be metered by the vacuum meter. At the time, the vacuum meter reaches its minimum metering limitation.
Similarly, the Pirani vacuum meter mentioned above measures the vacuum pressure according to the changes of temperature and thus the resistance of the filament resulted from the change of gas pressure. In general, the variation of resistance of the filament can be measured by a bridge circuit as shown in FIG. 17. In the utilization of the bridge circuit of FIG. 17 to measure the vacuum pressure, the bridge circuit should be zero-point calibrated in a first step by the following steps:
(1) Positioning the vacuum meter in a very low vacuum pressure (i.e., a very high-degree vacuum) which is extremely low in comparison with the minimum limitation of the meter, so that the very low vacuum pressure is called as a pseudo-absolute vacuum. At this time, it deems that the heat loss of the filament only resulting from the solid leads and the surface radiation of the filament; PA1 (2) Adjusting a voltage meter R.sub.2 ' of the bridge circuit to make the output of the bridge circuit to be nulled, i.e., the output of the bridge circuit is balanced. It means that an electrical response signal generated from heat exchanges of the solid heat-transfer and the surface radiative heat-transfer is deleted. The adjustment step is handled in a constant temperature (i.e., a reference temperature mentioned hereinafter); and PA1 (3) Filling the vacuum system with gas to a linear pressure region of the meter and then measure the output of the bridge circuit by the Pirani vacuum meter and a standard vacuum meter, respectively. Then calculate the sensitivity of the vacuum meter by means of at least the two measured values and store the calculated result into a memory for later uses.
The conventional Pirani vacuum meter, however, has a problem that the temperature and resistance of the filament will change with the ambient temperature so that an ambient temperature drift effect occurs. The ambient temperature drift effect will change the measured signal of the meter. To avoid such a problem, one conventional approach, as shown in FIG. 17, is to set a dummy tube S' which is similar to a real sensing tube G' in structure on one arm of the bridge-circuit for compensation. The real sensing tube G' is set on the other arm of the bridge circuit which is also shown in FIG. 17. The dummy tube S' is a sealed tube in a pseudo-absolute vacuum and thus will not be influenced by the vacuum pressure. Further, the temperature of an filament of the dummy tube S' is set to be the same as that of the sensing tube G'. Besides, the dummy tube and the sensing tube are very close each other so that the ambient temperatures thereof are very similar. Accordingly, the change of the ambient temperature will influence simultaneously the two tube but not the output of the bridge circuit. In reality, however, the construction and the spatial configuration of the two tubes would not be identical each other and the compensation of temperature could not be sufficiently perfect. Such method therefore can not thoroughly solve the problem of the ambient-temperature-drift effect. That is, the drifting noise generated by the ambient temperature will still influence the accuracy and the minimum limitation of the vacuum pressure measured by the vacuum meter.
The operation methods of the Pirani vacuum meter include: (1) a constant-bias method: the vacuum pressure is measured according to the voltage difference of the bridge circuit; since the resistances of most of metals will increase with the increase of the temperature and the temperature of filament will change proportionally to the vacuum pressure, the vacuum pressure can be measured by means of detecting the resistance of the filament (i.e., the voltage difference of the output of the bridge circuit); and (2) a constant-temperature or constant-resistance method: this method is to maintain the temperature of the filament by adjusting the bias-voltage of the bridge circuit (i.e., the resistance of the filament is constant and the bridge circuit is always kept balanced) and then to measure the vacuum pressure according to the applied power or the voltage drop on the filament. At present, most of the Pirani vacuum meters adopt the second operation method since the second method has a better sensitivity. In general, the filament is made of metals with higher temperature coefficient of resistance (TCR). Conventionally, the materials include Wu, Ni or other metal alloys.
The conventional thermal conductive-type vacuum meters, however, have large volumes and it is hard to make the respective temperatures of the vacuum meter and the dummy tube match well each other. The linear range of pressure of the vacuum meter therefore is only between 1 to 10.sup.-3 -10.sup.4 torr and is difficult to be lower. In addition to the above disadvantage, such vacuum meters are only manufactured in a single-unit production but not in a batch production which makes the cost of the meter expensive.
Recently, it is widely considered to utilize the semiconductor micromechining technology to manufacture various micro-sensors. Such method can fabricate the sensors in batch production and the volume of each of the sensors thus fabricated is very small. The technology also provide the advantage of manufacturing a signal-processing IC with the micro-sensor for versatile sensing. One of such micro-sensors is as shown in FIG. 18 taught by Mastorangele. The device comprises a polysilicon layer 181 which is made by the semiconductor technique and is floating on a substrate and a plurality of longitudinal slim beam 182 (only one is shown in the figure) made by an anisotropic etching method in replace of the conventional filament. The longitual slim beam 182 is used to make the vacuum meter having a high thermal resistance, but as the conventional filament, the surface area of the beam 182 is too small to provide sufficient striking chances of molecules for sensitivity. Thus, the vacuum meter is hardly used for measuring the pressure lower than 10.sup.-3 -10.sup.4 torr.
To add the sensing area, it has been disclosed a thermopile vacuum meter of which the floating plate has a larger surface area in comparison with that of the meter shown in FIG. 18. As shown in FIG. 19, the vacuum-meter comprises a floating glass plate 191 having one end connected to a semiconductor substrate 192 and the other end floating in a cavity of the substrate 192. A plurality of serial thermopile sensing elements 193 and heating elements 194 are installed on the surface of the floating glass plate 191. Although the meter has larger surface area, the cross area of the floating plate 191 contacting with the substrate 192 is large which results in a worse heat isolation. Further, the temperature of the floating plate in proximity to the end connected with the substrate 192 will rise slower than other positions so as to decrease the effective average temperature of the meter and thus the pressure sensitivity thereof is worse.